I don't think I'm particularly prudish. But after visiting Tate Modern's new Pop Life show, perhaps I've been deluding myself. Faced with Ilona's Asshole, a silkscreen measuring 2.5 metres by 1.5 metres, swiftly followed by a similarly sized image of the same woman inserting a dildo while her husband stimulates her, and another of – well, as they say in the Ealing comedies, I didn't know where to look.

A couple of rooms earlier, the label on the door of a long, thin red hallway warned me that the "images" inside were "challenging". Always up for a challenge, I entered, to find one image – that of a nude prepubescent Brooke Shields (the room hadn't yet been closed by the police when I visited). I looked briefly at the photo – so briefly that until all the recent hoo-hah I thought it was a bad, pre-Photoshop montage, as the child's head seemed hugely out of proportion to her weirdly shiny, Barbie-like body. I admit I spent rather more time reading the caption, unimpressed by the slippery curator-speak of "challenge", as if chosen to pre-empt my lack of interest. I then moved on to photographs of Cosey Fanni Tutti, a performance artist who posed for men's wank mags and displayed the results in the 1970s, to a similar uproar to the current Brooke Shields row (the police removed them, as pornography).

But what made me pause was the other viewers. This was press day, and it cannot be denied that a pack of journos wandering about with notebooks, solemnly standing in front of a photo of a woman masturbating in a lace bustier, is irresistibly comic. But I realised that, for the first time since I was a child, I was thinking about myself and my behaviour in a gallery. Galleries are for looking, not for being looked at. And yet with these works, it became very obvious that all the spectators were suddenly aware of being observed: viewees not viewers. Suddenly the gallery was filled with stiff, self-conscious shoulders and half-turned backs.

I suspect I wasn't the only one feeling uncomfortable, though. Most of the people there seemed to be trying to pretend there was no difference between graphic images of sex and an earlier Warhol portrait of Hockney, or Hirst's calf in a tank, elsewhere in the gallery. But of course there was: I had peered slantways from a few inches away at Martin Kippenberger's abstract 8. Preis to see what it was made of (lacquer and denim, as it turns out). You'd have to be pretty brave to attempt that with Koons's silkscreens.

A middle-age man made a note about one of Cosey Fanni Tutti's photos, and then looked over his shoulder, as if afraid his mother might be watching. Two people walked into the doorway of the Koons room, had a quick look around, read the caption, tittered quietly, rolled their eyes and left. In general, there was less talking in these rooms, much more of a sense of "let's get this looked at and get out of here".

I've spent decades looking at painted nudes. I've seen more rapes of Sabine women than I can count. Ditto nasty battle scenes, murders, decapitations (Salome, Judith and Holofernes, anyone?). So nudity, death, violence – at least in art – hold few surprises. Now, for the first time, I actually had to ask myself the question: how do I look at this? Insofar as it disoriented me, and made me reflect, I suppose the works had some value. But, in truth, I could have coped without seeing Ilona's asshole. Thanks for that, Jeff.

Tate Modern's decision to remove a naked 10-year-old Brooke Shields from display again highlights the moral and legal issues surrounding children in artworks. The old masters had no such problems, writes Laura Cumming

Adrian Searle: If Spiritual America is a comment on the commodification and premature sexualisation of Brooke Shields, who was complicit in turning her into a 10-year-old sex object? Not Richard Prince